The invention provides a method of producing plain carbon steel strip which has an excellent balance of ultimate tensile strength and elongation to break making it particularly suitable for the production of structural steel products. Strip produced in accordance with the invention may for example be used as a feed material that is hot dip coated with zinc or aluminium/zinc alloys to produce roof decking, guttering and other structural steel products.
The term “strip” as used in the specification is to be understood to mean a product of 5 mm thickness or less.
Recent developments in continuous casting techniques have included the casting of steel strip by continuous casting in a twin roll caster. In this technique molten metal is introduced between a pair, of contra-rotated horizontal casting rolls which are internally water cooled so that metal shells solidify on the moving rolls surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls, the term “nip” being used to refer to the general region at which the rolls axe closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also bean proposed. The casting of steel strip in twin roll casters of this kind is for example described in U.S. Pat. Nos. 5,184,668, 5,277,243 and 5,934,359.
We have determined that it is possible by continuous strip casting to produce strip which is highly susceptible to work hardening by cold rolling, ie. the ultimate tensile strength of the strip can be dramatically increased by moderate cold rolling. It has been further found that this work hardening effect is particularly pronounced in the case of silicon/manganese killed plain carbon steel, and increases with increasing manganese and silicon contents in the steel chemistry. Silicon/manganese killed steels are particularly suited to twin roll strip casting since aluminium killed or partially killed steels cannot be cast satisfactorily due to formation of solid inclusions which agglomerate and clog the fine flow passages in the metal delivery system of the caster to produce effects and discontinuities in the resulting strip product. A silicon/manganese killed steel will generally have a manganese content of not less than 0.20% (typically about 0.6%) by weight and a silicon content of not less than 0.10% (typically about 0.3%) by weight.
By an extensive test program we have determined that it is possible by cold rolling continuously cast plain carbon steel strip to produce a strip with an ultimate tensile strength of at least 680 MPa and an elongation-to-break in the range 8% to 12% which is an excellent balance of properties for use in many structural steel products such as roof decking and guttering.
So far as the applicants are aware, prior to the subject invention, it has not been possible to produce hot dip coated steel strip having this combination of properties from plain carbon steel and, as a consequence, it has been necessary to produce the steel strip from sore expensive grades of steel, such as low alloy steels which include specific additions of strengthening elements.
One known type of plain carbon steel strip that is used as a feed material for hot dip coating with aluminum/zinc alloys is produced by BHP Steel (JLS) Pty Ltd under the code name G550. G550 steel strip is produced by casting plain carbon steel slabs, hot rolling the slabs to form strip and thereafter coiling the strip, uncoiling and thereafter cold rolling the strip to a final product size of 0.25-2 mm, and heat treating the cold rolled strip to produce the final product. G550 steel strip has a guaranteed minimum ultimate tensile strength of 550 MPa and in a number of instances has ultimate tensile strengths above 700 MPa. For example, one commercially available G550 steel strip (Zincalume G550 coated steel) that is produced from plain carbon steel and is used for roof decking has an ultimate tensile strength of 680-780 MPa (based on a test sample of 0.42 mm thickness and an original gauge length of 80 mm). However, this G550 steel strip only has an elongation-to-break of 1-6%. The present invention enables production of a plain carbon steel strip of comparable tensile strength but an even better elongation-to-break.
According to the present invention there is provided a method of producing steel strip, comprising continuously casting plain carbon steel into a strip of no more than 5 mm thickness,                coiling the strip;        uncoiling the strip;        cold rolling the uncoiled strip; and        annealing the cold rolled strip to produce a stress relieved microstructure therein;        wherein the cold rolling produces a cold reduction in a range which is sufficient to increase the tensile strength of the strip to at least 680 MPa but such that the total elongation to break of the strip after said annealing is in the range 8% to 12%.        
The tensile strength of the strip may be at least 700 MPa.
The continuous strip casting step may be carried out by means of a twin roll strip caster.
The term “plain carbon steel” is understood to mean steel of the following composition, in weight percent:
C:0.02-0.08;Si:0.5 or less;Mn:1.0 or less;                residual/incidental impurities: 1.0 or less; and        Fe: balance.        
The term “residual/incidental impurities” covers levels of elements, such as copper, tin, zinc, nickel, chromium, and molybdenum, that may be present in relatively small amounts, not as a consequence of specific additions of these elements but as a consequence of standard steel making. By way of example, the elements may be present as a result of using scrap steel to produce plain carbon steel.
The term “residual/incidental impurities” excludes:                (a) amounts of the elements silicon and manganese outside the ranges given with definition of “plain carbon steel”; and        (b) amounts of elements, such as the elements listed in the preceding paragraph that are specifically added to the steel for the purpose of strengthening the steel.        
The plain carbon steel may be silicon/manganese killed and may have the following composition by weight:
Carbon0.02-0.08%Manganese0.30-0.80%Silicon0.10-0.40%Sulfur0.005-0.05%Aluminiumless than 0.01%A typical composition is as follows:
Carbon0.06%Manganese0.66%Silicon0.324Sulphur0.01%                Total oxygen content 60 ppm @ 1600° C.        
The cold rolling may produce a cold reduction of the strip thickness in the range 40% to 80%.
The annealing may produce the stress relieved microstructure with no more than 10% recrystallisation and an elongation-to-break of at least 10%.
The annealing temperature may be at least 450° C. and may be in the range 500° C. to 600° C.
Optionally, the continuously cast strip may be in-line hot rolled to reduce the thickness of the strip prior to coiling. It is desirable that the hot rolling produce a thickness reduction of no more than 40%.
In cases where the strip is hot rolled, it is desirable that the subsequent cold rolling produces a cold reduction of the strip in the range 40% to 60%.
The invention further provides a plain carbon steel strip having an ultimate tensile strength of at least 700 MPa and an elongation to break in the range of 8% to 12%.